1. Field of the Invention
The present invention relates to devices for controlled coupling of laser radiation to matter. More specifically, the present invention provides an apparatus and method for providing thrust using a laser directed to a reflector array. In application, the present invention provides a novel method for propulsion of aircraft and spacecraft. In additional applications, the present invention provides a novel method of diagnosing a beam from a high power laser, and also provides a method for generating a shock wave with a shapeable wavefront.
2. Description of Related Art
Following the launch of the Sputnik satellite several decades ago, a space race ensued in which the United States and the U.S.S.R. raced to be the first to send a man into space. After the U.S. won this race, in the late sixties the U.S. space program achieved remarkable success, landing the first man on the moon. Over the last decade, orbital flights and satellite communication have become almost commonplace. The United States's shuttle has been regularly sending up a crew of several astronauts for scientific experiments and delivery of payloads into orbit. The U.S.S.R. has maintained a space station, in which cosmonauts have resided for many months. The United States plans to build a permanent space station in orbit around the Earth. Furthermore, unmanned satellites have become an important part of the modern technology, providing communication channels for worldwide communication, and taking photographs from orbit for use in weather forecasting and military reconnaissance.
The rockets delivering the satellites and man into space have been, without exception, propelled by conventional chemical rockets which carry their energy stored in the form of the chemicals stored in tanks. The energy is released in an explosive chemical reaction directed to thrust the spacecraft into space. Typically, the rocket is built in several stages, each of which is jettisoned in sequence after its fuel is spent. Each of these stages is typically used only for one launch; therefore the cost of one launch is typically very large. Furthermore, the cost of the ground crew and the complicated systems demand a great deal of attention and testing before a launch, leading to considerable amount of time between launches, stretching into weeks or months.
There is a need for a low-cost launching system for small spacecraft as an alternative to expensive, time-consuming and dangerous chemical launching systems. One particular application for such a launching system is the space station, which will require a large amount of equipment to be sent into orbit, there to be assembled. If the shuttle were the sole launch system, the shuttle would have to make many expensive trips to deliver all the parts for the space station, at a large cost. Another application for low-cost launching system is in the "Brilliant Pebbles" approach to strategic defense currently being pursued at Lawrence Livermore National Laboratory (LLNL). In this approach, a number of small satellites are stationed into space to destroy incoming enemy missiles. The "Brilliant Pebbles" approach would greatly benefit from a low cost, reliable launching system. There is also a need for a low-cost launching system for many other applications including space habitat supply, deep space mission supply, nuclear waste disposal, and manned vehicle launching.
Alternative propulsion methods have been proposed, but so far none have been applied to practical rocket systems. Propulsion of space craft by lasers recently has received serious attention for its potential to provide a low-cost, safe launching alternative to conventional chemical rockets.
In a ground-based laser propulsion system, a large fixed laser supplies energy to propel a spacecraft into space. A laser system has at least two potential advantages: extreme simplicity of on-board engine equipment, and potentially high performance. Because much of the laser engine's thrust energy is provided from the ground-based laser, the spacecraft itself can be made much lighter than conventional rockets; thus saving a large percentage of the spacecraft's thrust for lofting a payload. Furthermore, the spacecraft itself can be manufactured very inexpensively, without the complex mechanical equipment necessary for conventional chemical systems, and re-use is feasible. By far the largest investment in a ground-based laser propulsion system is construction of the laser facilities; however once built, they can be operated for numerous launches at relatively small additional cost. The ground-based laser propulsion may operate alone to provide a sole source of thrust, or it may aid a conventional chemical system.
One proposed laser-based system is a double pulse planar LSD wave thruster. In that system, a first laser pulse ablates a solid (or liquid) propellant. The propellant vaporizes, providing thrust. A second laser pulse is applied to the vaporized material creating a plasma which provides additional thrust. An advantage of that system is that very simple thrusters are possible, possibly just a block of propellant, that have very simple nozzles, or even eliminate them completely. Furthermore, such thrusters produce thrust at an angle to the incident laser beam, and they can be remotely steered by controlling the beam profile. The guidance system may be entirely ground-based, eliminating the need for on-board guidance and control hardware, allowing very cheap disposable vehicles which could be mass produced. Furthermore, because the propellant exhaust velocity is not limited by its chemical energy content, laser propulsion thrusters can provide exhaust velocities several times higher than chemical rockets. However, this type of thruster is inefficient for flight in the Earth's atmosphere, and requires laser pulses of very high energy, which may be difficult to generate and transmit through the atmosphere.
The disadvantages can be overcome by concentrating the laser energy at the vehicle. Several laser-based systems have been proposed that use single reflector to concentrate laser energy, for example, the Apollo Lightcraft. A large cylindrical reflector forms the front of the spacecraft, which concentrates laser energy at a point to form form plasmas in the region surrounding the vehicle. These designs suffer from several disadvantages: they require large precision optics which are difficult to integrate with the vehicle's structure, and they must be precisely aligned with the laser. In other words, the energy conversion efficiency is very sensitive to the beam direction. Furthermore, the thrust direction cannot be controlled remotely by adjusting the laser beam, instead the vehicle must include mechanisms for active steering control.